The present invention generally relates to serial printers, and more particularly to a serial printer which prints an image on a recording sheet using the electrophotography technique by forming a toner image on the recording sheet by a recording drum.
Recently, in order to meet the demands to reduce both the cost and size of printers using the electrophotography technique, serial printers provided with a carriage that carries out the printing using the electrophotography technique have been developed. According to such a serial printer, the carriage is moved on a transfer unit in a direction perpendicular to a sheet transport direction of the recording sheet so as to transfer an image on the recording sheet, and the transferred image on the recording sheet is fixed by a fixing unit which has the form of a roller arranged in the sheet transport direction. There are now demands to improve the printing quality of such serial printers, and to also realize high-speed printing and color printing.
FIGS. 1A and 1B show the construction of a first conventional serial printer. FIG. 1A shows a plan view of a part of this first conventional serial printer, and FIG. 1B shows a cross section of a carriage of this first conventional serial printer.
A serial printer 11 shown in FIGS. 1A and 1B is proposed in a Japanese Laid-Open Patent Application No.61-152463, for example. A shaft 14 is arranged parallel to transport rollers 13a and 13b which transport a recording sheet 12 in a sheet transport direction. A carriage 15 is movable in a direction perpendicular to the sheet transport direction under guidance of the shaft 14, and this carriage 15 is driven by a driving motor (not shown). A fixing unit 16 having a width greater than the width of the recording sheet 12 is fixedly arranged on the downstream side of transport roller 13a in the sheet transport direction. A transfer unit 17 is arranged under the recording sheet 12 along a moving direction of the carriage 15.
An image bearing member 21 is provided in the carriage 15, and this image bearing member 21 rotates at a peripheral speed in synchronism with the movement of the carriage 15. The surface of the image bearing member 21 is uniformly charged by a charger 22, and an electrostatic latent image is formed on the surface of the image bearing member 21 by an exposing unit 23. The electrostatic latent image on the surface of the image bearing member 21 is visualized into a toner image by a developing roller 26 using a toner 25 of a developing unit 24. The toner image on the image bearing member 21 is transferred onto the recording sheet 12 by the transfer unit 17 which confronts the image bearing member 21 via the recording sheet 12, and the transferred image is fixed when it is transported to the position of the fixing unit 16.
FIG. 2 shows the construction of a carriage provided with a fixing unit. This carriage is proposed in a Japanese Laid-Open Utility Model Application No.61-145649, for example. A fixing unit 27 is provided inside a carriage 15. A cleaner 30 cleans the surface of the image bearing member 21 after the printing ends so as to remove the residual toner.
A fixing roller 28 which rotates in the same direction as the image bearing member 21 is provided in the fixing unit 27. A heat source 29 such as a halogen lamp is provided within the fixing roller 28 as a heating means. This fixing roller 28 is preheated to a predetermined temperature by the heat source 29 prior to the printing operation, and the temperature during the printing is controlled by detecting the temperature by a temperature detector (not shown) such as a thermistor. In other words, the fixing unit 27 is moved together with the image bearing member 21 and carries out the image fixing immediately after the image transfer.
The image transfer by the transfer unit 17 is carried out by applying a predetermined voltage across the transfer unit 17 and the image bearing member 21. Hence, a conductive member such as conductive rubber is formed on a substrate which is made of aluminum or the like, to form the transfer unit 17.
Although not shown, a fixing unit is provided within the carriage, and this fixing unit is provided with a non-contact type heat source that irradiates a heat ray on the recording sheet. The non-contact type heat source may be a halogen lamp using infrared ray or, a xenon lamp using flash fixing. According to the serial printer proposed in a Japanese Laid-Open Patent Application No.56-77167, for example, the image bearing member (recording drum) recedes from the recording sheet about a guide shaft of the carriage when transporting the recording sheet.
Next, a description will be given of a second conventional serial printer, by referring to FIG. 3. FIG. 3 shows a line type color printer 31.
In FIG. 3, a charger 33, an exposing unit 34, three developing units 35.sub.Y, 35.sub.M and 35.sub.C, and a transfer drum 37 are arranged around a photosensitive drum 32. A recording sheet is wrapped around the transfer drum 36 and is clamped thereon. The developing unit 35.sub.Y is filled with yellow toner, the developing unit 35.sub.M is filled with magenta toner, and the developing unit 35.sub.C is filled with cyan toner.
In other words, the photosensitive drum 32 is uniformly charged by the charger 33, and a cyan portion is exposed by the exposing unit 34 and a cyan toner image is formed on the photosensitive drum 32 by the developing unit 35.sub.C. This cyan toner image is transferred onto tile recording sheet on the transfer drum 37 by the transfer unit 36. A magenta toner image is developed by the developing unit 35.sub.M during the next rotation of the photosensitive drum 32, and a yellow toner image is developed by the developing unit 35.sub.Y during the second next rotation of the photosensitive drum 32. As a result, by the sequential transfer of the cyan, magenta and yellow toner images onto the recording sheet, a color image is printed on the recording sheet. The color image transferred onto the recording sheet is heated and fixed by a fixing unit (not shown).
Next, a description will be given of a third conventional serial printer, by referring to FIGS. 4A and 4B. FIG. 4A shows a plan view of a serial color printer 41, and FIG. 4B shows a cross section of the serial color printer 41. This serial color printer 41 is proposed in a Japanese Laid-Open Patent Application No.62-58277, for example.
In a carriage 42, chargers 44a and 44b and an exposing unit 45 are arranged around a photosensitive drum 43 which rotates in a direction perpendicular to the sheet transport direction of the recording sheet. In addition, developing units 46.sub.C, 46.sub.M and 46.sub.Y for the three colors which are cyan, magenta and yellow are arranged in parallel to each other in an axial direction of the photosensitive drum 43 so as not to interfere with each other.
In other words, the surface of the photosensitive drum 43 is divided into equal portions along the axial direction, and the developing units 46.sub.C, 46.sub.M and 46.sub.Y are arranged in the axial direction of the photosensitive drum 43 along the moving direction of the carriage 42. Hence, a plurality of color developing units are provided with respect to the surface of the photosensitive drum 43, and each color image can be formed on the recording sheet as the carriage 42 moves back and forth. The color image is formed by feeding the recording sheet by an amount corresponding to the width of the developing unit.
However, according to the first conventional serial printer shown in FIGS. 1A and 1B, the recording sheet 12 is fed intermittently, and there was a problem in that the printing speed is low particularly when the printing is made on a large recording sheet. In addition, the line type fixing unit 16 is provided independently outside the carriage 15. Because the length of the fixing roller amounts to the width of the recording sheet 12 even though the recording sheet 12 is transported intermittently, the contact time between the fixing roller and the recording sheet 12 having the toner image transferred thereon becomes greatly different between the part where the recording sheet 12 is stationary and the part where the recording sheet 12 is transported. This difference in the contact time introduces inconsistent fixing, and there was a problem in that the image quality of the printed image on the recording sheet 12 becomes poor. Generally, the fixing unit 16 fixes the toner image by a fixing roller which is heated to a temperature of approximately 180.degree. C., and there was also a problem in that discoloring of the recording sheet 12 occurs due to the heat and the recording sheet 12 becomes yellowish when the contact between the fixing roller and the recording sheet 12 is maintained for a relatively long time.
On the other hand, according to the carriage shown in FIG. 2, the fixing roller 28 is simply pushed against the conductive rubber of the transfer unit 17. For this reason, the fixing roller 28 cannot push against the transfer unit 17 with a large pressure in order to prevent marks or wrinkles from being formed on the recording sheet 12 by the pressure. As a result, it is necessary to improve the fixing strength by reducing the moving speed of the carriage 15 and increasing the heating temperature so as to increase the effective heating time. However, when the heating temperature and the heating time are set to levels such that the sufficiently large fixing strength is obtained, there were problems in that the moisture absorbency of the recording sheet 12 changes and the recording sheet 12 shrinks, thereby deteriorating the image quality.
According to the color printer 31 shown in FIG. 3, it is necessary to-carry out the process three or more times in order to print a color image. However, there were problems in that it is difficult to accurately position the recording sheet when it is clamped on the transfer drum 37 and it is difficult to detect the position of the recording sheet during the printing process. In addition, there was a problem in that the toner is easily scattered when the recording sheet having the toner image is turned a plurality of times during the printing process. Furthermore, it is necessary to provide three or more developing units having a width greater than or equal to the line width, and there was also a problem in that the size of the color printer 31 increases as the size of the recording sheet is increased.
On the other hand, according to the color printer 41 shown in FIGS. 4A and 4B, the plurality of developing units 46.sub.C, 46.sub.M and 46.sub.Y must be mounted with respect to the photosensitive drum 43 within the single carriage 42. In addition, since the printing is carried out as the carriage 42 moves back and forth, the photosensitive drum 43 is rotated in two directions. Hence, there were problems in that the photosensitive drum 43 is easily contaminated by the non-operating developing units, and the image quality is poor. In this case, it is possible to make the non-operating developing units recede from the photosensitive drum 43, similarly to a large color copying machine. However, it is difficult to provide a mechanism for making the plurality of developing units recede from the photosensitive drum 43 within the small carriage 42 that is used in the serial color printer 41.